A. Field of the Invention
The present invention relates to a method to determine the swing curve which will be used in photolithography of semiconductor technology. More particularly, the present invention relates to a method for determining a dark-to-clear exposure dose in a swing curve.
B. Description of the Related Art
Currently, the integration degree of large-scale integrated circuits is proceeding higher at a speed of a generation per three years. This high integration degree is gradually achieved along with the development of the fine processing technology, especially the technique of photolithography. In the semiconductor process, the main function of the process of photolithography is to define patterns and doping area on individual thin films formed on a semiconductor substrate.
Generally, the process of photolithography for a semiconductor wafer includes the following steps. At first, a photoresist film is coated on the surface of the semiconductor wafer. Next, using a mask with a pattern and an ultraviolet light as an exposing light source, the photoresist film is exposed such that the pattern is transferred from the mask to the photoresist film as a latent pattern. After the exposure, the photoresist film is developed to remove the exposed portion of the photoresist film.
In the process of photolithography, the object of the exposure step is to make the photoresist film absorb appropriate and sufficient exposure dose so as to perform the photochemical transformation. The factors for controlling the exposure dose are the intensity of the exposing light and the time period of the exposure. Furthermore, the magnitude of the exposure dose required to complete the photochemical transformation depends on both the thickness of the exposed photoresist film and the pattern size needed to be transferred.
In the semiconductor process, after the parameters of the process for the etching step is decided and set, it is usually necessary to determine or adjust the thickness of the photoresist film coated on the surface of the thin film. As for the process of photolithography, a swing curve as shown in FIG. 1 is typically adapted for determining the required magnitude of the exposure dose corresponding to a specific thickness of the photoresist film. Referring to FIG. 1, the swing curve is similar to a sine wave and depicted in terms of the magnitude of the exposure dose with respect to the thickness of the photoresist film. The magnitude of the exposure dose decides the degree of the photochemical transformation of the photoresist film. More specifically, the magnitude of the exposure dose is positively proportional to the degree of the photochemical transformation of the photoresist film.
As a method for obtaining the above-mentioned swing curve, a dark-to-clear exposure process has been disclosed. Hereafter, the conventional method will be described in detail with reference to FIGS. 2(A) and 2(B). FIG. 2(A) is a cross-sectional view showing the conventional method for obtaining the above-mentioned swing curve. Referring to FIG. 2(A), at the beginning of the dark-to-clear exposure process, a photoresist film 2 with a given thickness 3 is coated on a silicon dummy wafer 1. Subsequently, using a mask 4 without a pattern, the photoresist film 2 is exposed by means of step and repeat method using an exposing light 5. In other words, the surface of the photoresist film 2 is sectioned to a plurality of rectangular chips and repeatedly exposed from one chip to another. FIG. 2(B) is a plane view showing the photoresist film coated on the silicon dummy wafer shown in FIG. 2(A). Referring to FIG. 2(B), each of the chips 6 is exposed by different magnitudes of the exposure dose. After the completion of exposing all the chips 6, the photoresist film 2 is developed to remove the exposed portions. Next, based on the color appearing due to the light reflected from the surface of the silicon dummy wafer 1, each of the chips 6 is inspected by using human eyes to determine whether the photoresist film 2 coated thereon is completely removed. After the inspection, the magnitude of the exposure dose resulting in the complete removal of the photoresist film 2 is recorded.
Using the same steps as mentioned above, 24 silicon dummy wafers are sequentially exposed. It should be noted that the thicknesses of the photoresist films coated on the 24 silicon dummy wafers are different from each other. By this manner, the swing curve representing the relationship between the thickness of the photoresist film and the magnitude of the exposure dose for completely removing the photoresist film is obtained. However, for the thickness of the photoresist film at the order of .mu.m, the above-mentioned conventional method using the human eyes to inspect whether the removing of the photoresist is complete or not is very easy to make error. Therefore, it difficult to obtain an accurate swing curve for the .mu.m order's semiconductor technology.